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<p>声明具有以位为单位的明确大小的类数据成员。相邻的位域成员可以打包成共享和跨过各个字节。
</p><p>位域声明是使用下列声明符的<a href="data_members.html" title="cpp/language/data members">类数据成员声明</a>：
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">标识符</span><span class="t-mark">(可选)</span> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>:</b></code> <span class="t-spar">大小</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">标识符</span><span class="t-mark">(可选)</span> <span class="t-spar">attr</span><span class="t-mark">(可选)</span> <code><b>:</b></code> <span class="t-spar">大小</span>  <span class="t-spar">花括号或等号初始化器</span>
</td>
<td> (2)
</td>
<td> <span class="t-mark-rev t-since-cxx20">(C++20 起)</span>
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>位域的<i>类型</i>由<a href="declarations.html" title="cpp/language/declarations">声明语法</a>的 <span class="t-spar">声明说明符序列</span> 引入.
</p>
<table class="t-par-begin">


<tr class="t-par">
<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
</td>
<td> -
</td>
<td> 可选的任何数量<a href="attributes.html" title="cpp/language/attributes">属性</a>的序列
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">标识符</span>
</td>
<td> -
</td>
<td>  被声明的位域名。名字是可选的：无名位域引入指定数量的填充位
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">大小</span>
</td>
<td> -
</td>
<td>  值大于或等于零的<a href="constant_expression.html#.E6.95.B4.E5.9E.8B.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F" title="cpp/language/constant expression">整型常量表达式</a>。大于零时，这是位域将占有的位数。值零仅对无名位域允许使用并具有特殊含义：它指定类定义中的下个位域将始于分配单元的边界。
</td></tr>
<tr class="t-par">
<td>  <span class="t-spar">花括号或等号初始化器</span>
</td>
<td> -
</td>
<td>  此位域所使用的<a href="data_members.html#.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96" title="cpp/language/data members">默认成员初始化器</a>
</td></tr></table>
<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
<p>位域中的位数设置其所能保有的值的极限：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
 <span class="co1">// 三位的无符号位域，</span>
 <span class="co1">// 允许值为 0...7</span>
 <span class="kw4">unsigned</span> <span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">3</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    S s <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">6</span><span class="br0">}</span><span class="sy4">;</span>
    <span class="sy2">++</span>s.<span class="me1">b</span><span class="sy4">;</span> <span class="co1">// 在位域中存储值 7</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">b</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="sy2">++</span>s.<span class="me1">b</span><span class="sy4">;</span> <span class="co1">// 值 8 不适合此位域</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> s.<span class="me1">b</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 正式来说是由实现定义的行为，通常为 0</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">7
0</pre></div></div> 
</div>
<p>多个相邻位域通常打包在一起（尽管此行为是实现定义的）：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="co1">// 将通常占用 2 字节：</span>
    <span class="co1">// 3 位：b1 的值</span>
    <span class="co1">// 2 位：不使用</span>
    <span class="co1">// 6 位：b2 的值</span>
    <span class="co1">// 2 位：b3 的值</span>
    <span class="co1">// 3 位：不使用</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> b1 <span class="sy4">:</span> <span class="nu0">3</span>, <span class="sy4">:</span> <span class="nu0">2</span>, b2 <span class="sy4">:</span> <span class="nu0">6</span>, b3 <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> sizeof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 通常打印 2</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2</pre></div></div> 
</div>
<p>特殊的零大小无名位域可用于强行打破填充。它指定下个位域始于其分配单元的开始：
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="co1">// 通常将占用 2 字节：</span>
    <span class="co1">// 3 位：b1 的值</span>
    <span class="co1">// 5 位：不使用</span>
    <span class="co1">// 6 位：b2 的值</span>
    <span class="co1">// 2 位：b3 的值</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> b1 <span class="sy4">:</span> <span class="nu0">3</span><span class="sy4">;</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> <span class="sy4">:</span><span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// 开始新字节</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> b2 <span class="sy4">:</span> <span class="nu0">6</span><span class="sy4">;</span>
    <span class="kw4">unsigned</span> <span class="kw4">char</span> b3 <span class="sy4">:</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> sizeof<span class="br0">(</span>S<span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// 通常打印 2</span>
<span class="br0">}</span></pre></div></div>
<p>可能的输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">2</pre></div></div> 
</div>
<p>若位域所指定的大小大于其类型的大小，则值由类型所限制：<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/integer.html"><span class="kw120">std::<span class="me2">uint8_t</span></span></a> b <span class="sy4">:</span> <span class="nu0">1000</span><span class="sy4">;</span></span></span> 仍会保有 0 与 255 间的值。额外位成为不使用的填充位。
</p><p>因为位域不必然始于一个字节的开始，故不能取位域的地址。指向位域的指针和非 const 引用是不可行的。从位域<a href="reference_initialization.html" title="cpp/language/reference initialization">初始化 const 引用</a>时，将创建一个临时量（其类型是位域的类型），并以该位域的值复制初始化，而引用绑定到该临时量。
</p><p>位域的类型只能是整型或枚举类型。
</p><p>位域不能是<a href="static.html" title="cpp/language/static">静态数据成员</a>。
</p><p>没有位域<a href="value_category.html" title="cpp/language/value category">纯右值</a>：左值到右值转换始终生成位域底层类型的对象。
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx20"><td>
<p>位域无<a href="data_members.html#.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96" title="cpp/language/data members">默认成员初始化器</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">1</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">1</span> <span class="br0">{</span><span class="nu0">0</span><span class="br0">}</span></span></span> 非良构。
</p>
</td>
<td><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td></tr>
<tr class="t-rev t-since-cxx20"><td>
<p>在位域大小与默认初始化器有歧义的情况下，选择组成合法大小的最长记号序列：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> a<span class="sy4">;</span>
  <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
  <span class="kw1">struct</span> S <span class="br0">{</span>
    <span class="co1">// 简单情况</span>
    <span class="kw4">int</span> x1 <span class="sy4">:</span> <span class="nu0">8</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>               <span class="co1">// OK；"= 42" 是 花括号或等号初始化器</span>
    <span class="kw4">int</span> x2 <span class="sy4">:</span> <span class="nu0">8</span> <span class="br0">{</span> <span class="nu0">42</span> <span class="br0">}</span><span class="sy4">;</span>             <span class="co1">// OK；"{ 42 }" 是 花括号或等号初始化器</span>
    <span class="co1">// 歧义</span>
    <span class="kw4">int</span> y1 <span class="sy4">:</span> <span class="kw2">true</span> <span class="sy4">?</span> <span class="nu0">8</span> <span class="sy4">:</span> a <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>    <span class="co1">// OK；不存在 花括号或等号初始化器</span>
    <span class="kw4">int</span> y2 <span class="sy4">:</span> <span class="kw2">true</span> <span class="sy4">?</span> <span class="nu0">8</span> <span class="sy4">:</span> b <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>    <span class="co1">// 错误：不能赋值给 const int</span>
    <span class="kw4">int</span> y3 <span class="sy4">:</span> <span class="br0">(</span><span class="kw2">true</span> <span class="sy4">?</span> <span class="nu0">8</span> <span class="sy4">:</span> b<span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>  <span class="co1">// OK；"= 42" 是 花括号或等号初始化器</span>
    <span class="kw4">int</span> z <span class="sy4">:</span> <span class="nu0">1</span> <span class="sy3">||</span> new <span class="kw4">int</span> <span class="br0">{</span> <span class="nu0">0</span> <span class="br0">}</span><span class="sy4">;</span>    <span class="co1">// OK；不存在 花括号或等号初始化器</span>
  <span class="br0">}</span><span class="sy4">;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td></tr>
</table>
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
<p>位域的下列性质<i>由实现定义</i>
</p>
<ul><li> 以范围外的值对有符号位域进行赋值或初始化，或对有符号位域进行自增越过其范围，所产生的值。
</li><li> 任何关于类对象中位域的实际分配细节
</li></ul>
<dl><dd><ul><li> 例如，在某些平台上，位域不跨过字节，其他平台上会跨过
</li><li> 还有，在某些平台上，位域从左到右打包，其他为从右到左
</li></ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-until-cxx14"><td>
<ul><li> 未显式标为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span></span></span> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">short</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span>、<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">long</span></span></span> 位域是有符号还是无符号。
</li></ul>
<dl><dd><ul><li> 例如，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> b<span class="sy4">:</span><span class="nu0">3</span><span class="sy4">;</span></span></span> 可能有值域 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">0</span>..<span class="nu0">7</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="nu0">4</span>..<span class="nu0">3</span></span></span>。
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-until-cxx14">(C++14 前)</span></td></tr>
</table>
<p>在 C 编程语言中，位域的宽度不能超过底层类型的宽度。
</p>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011): 
</li></ul>
<dl><dd><ul><li> 9.6 Bit-fields [class.bit]
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998): 
</li></ul>
<dl><dd><ul><li> 9.6 Bit-fields [class.bit]
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

<tr class="t-dsc">
<td>  <div class="t-dsc-member-div"><div><a href="../utility/bitset.html" title="cpp/utility/bitset"> <span class="t-lines"><span>bitset</span></span></a></div></div>
</td>
<td>  实现常量长度的位数组 <br> <span class="t-mark">(类)</span> </td></tr>
</table>

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